1. Field of the Invention
The invention relates to a flow rate control valve provided in an internal combustion engine equipped with a variable mechanism, which operates a movable member in accordance with the supply/discharge of a hydraulic fluid and thus makes a valve opening/closing characteristic of an engine valve variable, to control the valve opening/closing characteristic.
2. Description of Related Art
Generally, many internal combustion engines are equipped with a variable valve timing mechanism that varies the timing of the engine valves such as intake valves and exhaust valves to improve fuel economy, enhance output, and the like. In such internal combustion engines, a movable member of the variable valve timing mechanism, which is fastened to one end of a camshaft by a bolt, is operated through the supply and discharge (supply/discharge) of a hydraulic fluid to and from the variable valve timing mechanism to change the rotational phase of the camshaft relative to a crankshaft, thereby varying the valve timing of the engine valves.
The aforementioned supply/discharge of the hydraulic fluid is controlled through the driving of a flow rate control valve (an oil control valve) that includes a housing and a spool. The housing is disposed across a plurality of oil passages through which the hydraulic fluid is supplied/discharged to/from the variable valve timing mechanism. The housing includes an accommodation chamber, and a plurality of ports, through which the accommodation chamber communicates with the oil passages respectively, at a plurality of locations in a direction along an axis. A spool provided in the accommodation chamber may reciprocate in the axial direction of the accommodation chamber. The respective ports are then opened or closed based on the position of the spool in the axial direction of the accommodation chamber, the amounts of the hydraulic fluid supplied to and discharged from the variable valve timing mechanism are thereby adjusted, and the movable member is moved.
Meanwhile, in the variable valve timing mechanism, it is desirable to enhance the responsiveness in operating the variable mechanism and suppress the leakage of oil from the oil passages between the variable mechanism and the flow rate control valve. Accordingly, the flow rate control valve is ideally disposed in a central region of the variable valve timing mechanism, which shortens the oil passages therebetween.
As described in Published Japanese Translation of PCT Application No. 2009-515090 (JP-A-2009-515090), it is conceivable to employ as the aforementioned housing a bolt (a valve housing) for fastening a movable member (an output element) of a variable valve timing mechanism (a device for variably adjusting the control time of a gas exchange valve) to a camshaft, and endow this bolt with the function of a flow rate control valve (a control valve). It should be noted that the terms in parentheses following the names of the members are used in Published Japanese Translation of PCT Application No. 2009-515090 (JP-A-2009-515090).
In this case, a spool (a control piston) is accommodated in the bolt movably in a reciprocating manner in a direction along an axis. Various ports (an input port, a work port, and an output port) for supplying/discharging the hydraulic fluid to/from the variable valve timing mechanism are formed through the bolt. The spool is moved in the axial direction of the housing, so that the respective ports are opened or closed or the areas of communication (opening degrees) of the respective ports are changed. As a result, the amounts of the hydraulic fluid supplied to and discharged from the variable valve timing mechanism are adjusted.
Because the bolt is located in the central region of the variable valve timing mechanism, the flow rate control valve is near the variable valve timing mechanism. The oil passages for the hydraulic fluid between the flow rate control valve and the variable valve timing mechanism are short, and the areas of faces to be sealed are small. Consequently, responsiveness is enhanced and leakage of oil is suppressed.
However, if the bolt is screwed to the camshaft to fix the movable member to the camshaft, the bolt may become distorted by a fastening torque as a result of a manufacturing error of the movable member, an assembly error of the movable member, manufacturing errors of the bolt and the camshaft, or the like. Distortions of the bolt may result in a great dispersion of the gap between the bolt and the spool in some locations, thereby altering the flow rate characteristic of the flow rate control valve or cause an operational failure in the spool.
In this view, in the aforementioned Published Japanese Translation of PCT Application No. 2009-515090 (JP-A-2009-515090), an inner peripheral region of the bolt is constituted by a sleeve (a press medium guide insert) as a separate member. Each of the bolt and the sleeve is provided, at a plurality of locations along the axis, with a plurality of ports through which the accommodation chamber communicates with the oil passages respectively. The bolt and the sleeve together constitute the housing of the flow rate control valve.
According to the aforementioned Published Japanese Translation of PCT Application No. 2009-515090 (JP-A-2009-515090), the sleeve is interposed between the bolt and the spool. Thus, while the bolt is in charge of the fastening function of the housing of the flow rate control valve, the sleeve and the spool are in charge of the valve function of the housing of the flow rate control valve. The separate members are in charge of both the functions respectively. Therefore, the sleeve and the spool are not affected by the fastening torque of the bolt, and unlikely to be distorted.
However, in the above-described flow rate control valve with the sleeve constituting part of the bolt (the inner peripheral region thereof), the sleeve may be assembled with the bolt with the corresponding ports of the sleeve and the bolt deviant from each other in a circumferential direction respectively. In addition, the sleeve assembled with the bolt may rotate with respect to the bolt due to vibrations or the like of the internal combustion engine, and the ports of the sleeve may deviate from the ports of the bolt in the circumferential direction respectively. Then, if the respective ports are closed due to this distortion, it is difficult to ensure a flow rate required for the supply/discharge of the hydraulic fluid.